Fixed wing aircraft controls typically take the form of secondary surfaces such as flaps which are hinged to the trailing or aft portion of a lifting or directional member on the aircraft. Such members may include ailerons; elevators; rudders; and the like on airplanes or missiles. The attitude control flaps are generally hinged to the wing or other member near the leading edge of the control flap so as to produce an aeromechanically stable wing/flap assembly relative to the remainder of the wing. Pressures exerted on the control flap by the airstream through which it moves will tend to hold the flap in its neutral position when hinged in this way. The orientation of the control flap relative to the remainder of the wing is changed by means of an hydraulic or pneumatic system which is operated by the pilot or autopilot. This tendency of the airstream to move the control flap to a stable neutral position occurs only when the flap is hinged to the wing at a location near the leading edge of the control flap.
In earlier days of aviation, before the hydraulic or pneumatic fluid power boost systems were available for aircraft, the control flaps on aircraft wings, for example, were effectively hinged to the wing at a location which was rearwardly offset from the flap leading edge --actually or in effect-- so as to produce aerodynamic balance in the flaps, since pressures forward of the hinge line would tend to counteract pressures acting on the flap rearward of the hinge line. In such early prior art systems, too much aerodynamic balance had a tendency to cause the control flaps to be unstable in any fixed deflected position because the airstream impinging on the leading edge of the control flap tends to impart a further pivoting moment to the control flap which will tend to deflect the control flap out of whatever pivotal position it is in. To lessen the control forces required to move a stable hinge arrangement, the early prior art control flaps were often provided with trim tabs on their trailing edges, which tabs could be mechanically pivoted on the flaps so as to counteract the tendency of the airstream to return a deflected, stably hinged control surface to a neutral position. Trim tabs were set by the pilot by means of a slow acting actuator, such as a screw, or the like, so as to achieve a desired deflected position with zero or small pilot control force. The objective of either including trim tabs or the rearward offset of the control flap hinge line --or both-- was to lessen the forces needed to pivot the control flaps, but in either case the prior art had to take care to make the control surface stable in the presence of the energetic airstream.